The invention relates to a method for the protection of metal heating electrodes of melting furnaces against reduction of ions, particularly of cations, contained in a silicate melt, by the use of a DC current, wherein the positive pole of the DC current source is connected to the heating electrodes and the negative pole is connected to at least one auxiliary electrode. The invention also relates to an arrangement for performing this method, comprising metal heating electrodes connected to a source of AC current and to the positive pole of a source of AC current and to the positive pole of a source of DC current and comprising also at least one auxiliary electrode connected to the negative pole of the source of DC current.
Electric melting of silicates by direct passage of electric current through the melt is at present generally accomplished with the use of metal heating electrodes, especially molybdenum electrodes.
If the melt contains some cations, for instance cations of lead, which is a component of lead crystal glass, of arsenic and antimony, used for melting of glass as clarifying agent, of iron, contained for instance in basalt, of nickel used for black enamels or of copper, chromium or manganese used as color admixtures to glass melts, a reaction with the metal of the heating electrode occurs and, according to the concentration of the cations in the melt and to melting conditions, even to the separation of the cations on the heating electrodes. The material of the heating electrodes is corroded by this reaction and the deposited metal flows to the bottom of the furnace, where it accumulates, corrodes the bottom and can even depreciate the melt by color striae. This reaction can also cause an increased formation of bubbles. If the concentration of these cations, for instance of Pb.sup.+.sup.2, in the melt is higher, the corrosion and the separation in metal shape can be so intensive that the electric melting becomes ineffective in operation. Alternatively, electrodes other than metal electrodes may be used, but this solution to the problem has certain disadvantages.
An electrochemical method is known in which tubes, cables, ships and similar objects are protected against corrosion by cathodic polarisation. The protected parts are in this case conductors of electrons and the electrolytic properties of the solution and the whole course of corrosion are well known.
Methods are also known for the protection of heat resistant furnace materials, which are conductors of ions. According to the French Pat. No. 994,796, the positive pole of a DC source is connected to a heat resistant wall of a melting aggregate and the negative pole is connected to graphite heating electrodes. According to the French Pat. No. 1,126,690, it is possible to protect parts which come in contact with the glass mass, such as passage ways, floaters, stirring elements, dosing elements and the like against corrosion by the glass melt by superposing DC current on the AC heating current, whereby the negative pole of the DC current is connected to the protected part and the positive pole is connected to one or more auxiliary electrodes of platinum or other material which is resistant to the glass melt. Another solution is disclosed in French Pat. No. 1,277,999, wherein conditions are created so that a polarisation current flows between the heat resistant material and the auxiliary electrode of platinum, molybdenum or steel, namely from this electrode to the protected heat resistant body. The method disclosed in German Democratic Republic Pat. No. 64,845 utilizes the fact that a transient layer wth a rectifying property is created if a DC source is connected between the heat resistant material and an auxiliary metal electrode, so that the electric current can pass in one direction only, namely from the melt to the heat resistant material. If the source of DC current is connected in opposite direction to this rectifying layer, i.e., the positive pole to the heat resistant material and the negative pole to the auxiliary metal electrode, only a small current can pass. The small current is sufficient, however, to shift the voltage of the heat resistant material to a zone with small corrosion. When the polarity is opposite, a protective effect is equally obtained, since two opposite flows of the glass mass are mutually compensating in the border layer. That is, a conventional normal glass flow occurs due to the heat gradient and a conventional flow of the glass mass occurs due to polarisation of the heat resistant material at a low current density of 3 mA/cm.sup.2.
These methods assure that the protected parts are provided with coatings of conductive material, for instance of metal, graphite, MgO, SiC and similar and are particularly suitable for protection of sufficiently conductive materials, particularly of electrically molten cast refractory materials having an Al.sub.2 O.sub.3, ZrO.sub.2 base. In addition, at current densities above 1 mA/cm.sup.2, a corrosion of the refractory material occurs at the region of contact of metal conductors with this material, due to the electric current, and in the region of the auxiliary electrodes with the glass melt an electrolytic decomposition of the melt occurs which appears in the form of bubbles. This problem is eliminated by the method of protection of fireclay, according to the Czechoslovak Pat. Nos. 136,876 and 136,877, with a current of a density below 1 mA/cm.sup.2, whereby the conductive elements are connected to a transition layer created on the heat resistant material at its contact with the melt. A method according to Czechoslovak Pat. No. 132,369 protects the heat resistant materials with an external metal coating connected to ground.
These methods have as an object the prevention of any dissolution of oxides in the heat resistant material, and their release into the melt. None of these methods is, however, capable of preventing a reduction of ions, particularly of cations, from the melt on heating electrodes.
Methods of protection of electrodes by DC current are also known. According to French Pat. No. 982,980, the electrodes are connected to a source of AC current and to a DC circuit, whereby they act as anodes, The DC circuit is connected to prevent passage of AC current. This method is suitable when graphite electrodes are used, and its object is the prevention of coloring of the glass mass, particularly of borosilicate glass, by the electrodes. In a further known method of protection of heating electrodes, according to the U.S. Pat. No. 3,530,221, one pole of the source of DC current is connected to a system of electrodes made of a material resistant to anions and the other pole is connected to a system of electrodes resistant to cations. Positive electrodes are for instance of tin dioxide or platinum, negative electrodes of molybdenum or graphite. Even this method cannot prevent a reduction of ions on metal heating electrodes.